1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which perform imaging with applying a desired RF pulse, such as a fat saturation pulse, for controlling an image contrast.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on NMR signals generated due to the excitation.
In the field of magnetic resonance imaging, the techniques to control an image contrast include the fat suppression methods. The fat suppression methods that has been widely used in general include the CHESS (chemical shift selective) method, the SPIR (spectral presaturation with inversion recovery) method (also referred as SPECIR method), and the STIR (short TI inversion recovery) method.
Of the fat suppression methods, the CHESS method is referred as a frequency-selective fat suppression method since the method suppresses only fat signals frequency-selectively using the fact that the resonance frequencies of the water proton and the fat proton mutually differs by 3.5 ppm (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 7-327960, Japanese Patent Application (Laid-Open disclosure) No. 9-182729 and Japanese Patent Application (Laid-Open disclosure) No. 11-299753). A CHESS pulse is applied as a RF pre-pulse in advance of data acquisition for imaging in the CHESS method.
The SPIR method is also a frequency-selective fat suppression method which uses the difference in the resonance frequency between the water proton and the fat proton (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 2006-149583). In the SPIR method, a SPIR pulse that is a frequency-selective inversion RF pulse matched with a resonance frequency of fat signals is applied as a RF pre-pulse.
Meanwhile, the STIR method is a fat suppression method which uses a difference in T1 relaxation time between a fat signal and a water signal and a frequency-nonselective fat suppression method.
FIG. 1 is a time chart of the conventional pulse sequence under the FSE (fast spin echo) method with applying a frequency-selective fat saturation pulse as a RF pre-pulse.
In FIG. 1, RF denotes RF pulses, Gss, Gro and Gpe denote axes to which gradient magnetic field for slice selection, gradient magnetic field for RO (readout) and gradient magnetic field for PE (phase encode) are applied respectively, ECHO denotes echo signals.
As shown in FIG. 1, an α° frequency-selective fat saturation pulse RFc1 for suppressing unnecessary signals from fat is applied as a RF pre-pulse prior to a FSE sequence for imaging. In addition, a spoiler gradient magnetic field Gsp1 is applied in a gradient magnetic field direction for slice selection subsequently to the α° frequency-selective fat saturation pulse RFc1.
In the FSE sequence, a flip pulse RFI1 with 90 degrees of FA (flip angle) is generally applied as a RF excitation pulse. In addition, plural refocus pulses RFI2, RFI3, RFI4, . . . are applied at an ETS (Echo Train Space) subsequently to the flip pulse RFI1. Each FA of the refocus pulses RFI2, RFI3, RFI4, . . . is generally set to 180 degrees. An interval between the flip pulse RFI1 and the first refocus pulse RFI2 is set to ETS/2.
Meanwhile, a slice selection gradient magnetic field pulse Gss1 corresponding to the flip pulse RFI1, and slice selection gradient magnetic field pulses Gss2, Gss3, Gss4, . . . corresponding to the refocus pulses RFI2, RFI3, RFI4, . . . , respectively are applied. The slice selection gradient magnetic field pulse Gss1 corresponding to the flip pulse RFI1 has a dephasing part. Each of the slice selection gradient magnetic field pulses Gss2, Gss3, Gss4, . . . corresponding to the refocus pulses RFI2, RFI3, RFI4, . . . , respectively has spoiler gradient magnetic field parts on its both sides.
Further, readout gradient magnetic field pulses Gro2, Gro3, . . . each having a same area S are applied following the refocus pulses RFI2, RFI3, RFI4, . . . , respectively. In addition, a readout gradient magnetic field pulse Gro1 for dephasing is applied following the flip pulse RFI1. The area of the readout gradient magnetic field pulse Gro1 for dephasing is set to be S/2 which is half of each area S of the readout gradient magnetic field pulses Gro2, Gro3, . . . applied subsequent to the refocus pulses RFI2, RFI3, RFI4, . . . .
Moreover, phase encode gradient magnetic field pulses Gpe1, Gpe2, Gpe3, Gpe4, . . . having reversed signs and equal areas are applied in intervals between respective applications of the refocus pulses RFI2, RFI3, RFI4, . . . .
In the foregoing pulse sequence, echo signals Echo1, Echo2, . . . are generated by application of the readout gradient magnetic field pulses Gro2, Gro3, . . . .
In recent years, the high magnetization technique in the MRI apparatus has been investigated and a high magnetic field apparatus have been produced. However, especially under a high magnetic field not less than 3 T, it is known that there is a B1 inhomogeneity problem that an inhomogeneity in a RF magnetic field increases due to attenuation of a RF pulse since a RF pulse having a shorter wavelength attenuates more in a living body while a resonance frequency becomes higher. The B1 inhomogeneity is also referred as RF magnetic field inhomogeneity.
Consequently, an adequate fat suppression effect might not be achieved by simply applying a frequency-selective fat saturation pulse, such as a CHESS pulse, as a RF pre-pulse, depending on an imaging condition as in the case of being under a high magnetic field.
This problem is common to an imaging with application of a RF pulse for controlling an image contrast, as well as a fat saturation pulse. That is, a desired image contrast could not be obtained simply by applying a RF pre-pulse for controlling an image contrast.
On the other hand, an imaging with application of a RF pre-pulse has a problem with increasing an imaging time. Especially in multi-slice imaging, the problem is that the minimum TR (repetition time) is increased by application of a RF pre-pulse and to increase the number of slices becomes difficult. Note that, the minimum TR is a TR for imaging a specific slice set consisting of multiple slices.